1. Field of the invention
This invention relates to a servo motor control device employed in a positioning control system using a servo motor, such as an NC machining system or industrial robots, and more particularly to such a servo motor control device wherein a servo motor is controlled by means of a position feedback signal and a speed feedforward signal.
2. Description of the prior art
A feedback control system has been employed in the positioning control system of the industrial robot, for example. This control system performs the positioning control, calculating the deviation between a target position and a current position.
The above-mentioned feedback control is usually divided into stages of position loop, speed loop and electric current loop. In the stage of the position loop, the deviation between the target position of the robot and its current position is calculated and a result obtained from the calculation is multiplied by a position loop transfer function so that a speed command is obtained. In the stage of the speed loop, the deviation between a value of the speed command obtained in the stage of the position loop and the current speed is calculated. A result obtained from the calculation is multiplied by a speed loop transfer function so that a torque command is obtained. In the stage of the electric current loop, the deviation between a value of the torque command obtained in the stage of the speed loop and an actual value of current flowing into the servo motor is calculated. A result obtained from the calculation is multiplied by a current loop transfer function so that a current command is obtained. The current flowing into the servo motor is controlled by the obtained current command. The control in each of the stages is performed based on command values obtained from combination of proportional, integral, differential and other control manners.
A general purpose servo driver is usually used for the speed and current loops. Accordingly, a control device is composed of the servo driver and a position control section delivering a speed command signal based on the position feedback.
A target speed in the case where an arm of the robot is moved is shown by a trapezoidal smooth curve as shown in FIG. 5, for example. This target speed is integrated and the result is supplied as a target position command to the position control section. In order that a sudden change in the speed is avoided, acceleration at the respective times of start and termination of positive and negative accelerations is smoothly increase and reduced, thereby preventing occurrence of vibration. Thus, a settling time for positioning the robot arm at the target position can be shortened.
However, the response of the servo motor depends upon the positional deviation in the control by the position feedback. When the position feedback is performed by the proportional control, for example, time constants of the machine, speed loop and position loop and the time lag of the servo driver in its control cause a speed feedback waveform to respond with a time lag relative to the target speed command value, as is shown in FIG. 8. Consequently, a response time is disadvantageously increased.
A speed feedforward control has been recently performed for the purpose of improvement of the above-described responsibility. In the speed feedforward control, the value of the target speed command is added to the speed command value to the servo driver. However, if the feedforward control is being performed when a gain of the loop of the position control is increased for improvement of the responsibility, a controlled variable in accordance with the positional deviation at that time is added to the amount of feedforward based on the target speed command value and the result of this addition becomes the speed command value for the servo driver. Accordingly, an amount of positional deviation is increased as the result of time lag in response in the system after the servo driver. Consequently, the speed command deviates from the target speed command value at the time of acceleration so that the speed command becomes larger than the target speed command value. Thus, there is a problem that the waveform of the speed command value supplied to the servo driver deviates from the waveform of the target speed command value in the speed feedforward control.
As described above, in the case of the speed feedforward, a lag element in the system after the servo driver increases the positional deviation at the times of acceleration and deceleration. Accordingly, as shown in FIG. 9, the waveform of the speed command value for the servo driver deviates from the waveform of the target speed command value. Consequently, the vibration is produced and the settling time is increased.